A man – convinced his 88-year-old grandmother was trying to poison his Christmas Eve dinner – went into a mad frenzy in her Brooklyn apartment last night and stabbed her dozens of times, killing her, police sources said.

Rebecca Philpotts, was fixing dinner for her grandson, identified by sources as Alexander Philpotts, 25, in her apartment on Eastern Parkway in Crown Heights when he attacked her with two carving knives, sources said.

After mercilessly stabbing the woman more than 30 times, sources said, Philpotts called 911 to say that he had punched his grandmother, a retired nurse, then killed her with a knife.

“She’s poisoning my food,” he told the operator. “He said he ‘had to do it,’ ” a police source said.

When cops arrived, they found Rebecca laying in pool of blood in her bathroom and Alexander waiting for them.

He was immediately taken into custody.

Charges were still pending against him early today.

Alexander, who lives in Far Rockaway, had gone to his grandmother’s house for a Christmas Eve visit.

Nobody could explain what caused him to snap and suddenly decide the elderly woman was trying to kill him.